Because Hitler actually tried to exterminate several races of people. All of the others had the excuse of running a regime to make it 'better for the ordinary people'. But Hitler openly tried to kill Jews, Pols, Gypsies etc…

stalin was worse than hitler.

Very simply, Hitler tried to kill the Jews and the Jews control the media. They have made Hitler the worst one because he was on to them and they want to be seen as the biggest victims. The fact that millions of non-Jews died under Nazi rule means nothing. Look at how much money Israel and the Jews have squeezed out of Germany and the West. Try getting a fraction of that for Africans, Indians, Ukrainians, or any group that suffered under tyranny and/or oppression. Jews cry the most and get the most.

stalin did get a lot of real bad press, the main problem is that he killed most if not all members of the media in the common purges of that era or sent them to live in the gulags of siberia, at any length, the results were the same, dead men dont tell a lot of secrets that the powers of the realm dont want sent out. bad and atrocious leaders in history are a dime a dozen and you forgot idi amin and a whole bunch of bad guys in africa, and thats still going on as this is written, and there are more, just keep reading

Here are my thoughts. Stalin, like the others you mention, was not from the Anglo-Saxon culture. Hitler was viewed as more "like us" and hence, his evils seemed to be magnified.
How could someone with whom we shared so much in common with be so unremittingly evil?
We have to remember that World War II took place at a time when our culture had little to share with Ancient Romans, Chinese, Cambodians, Wallachians, Mongols, Huns and Macedonians. Hitler as an almost "brotherly " figure seemed particularly odious.

Again, these are just some thoughts of my own.

Even though Stalin murdered between 30 to 50 million people, while Hitler murdered 17-20 million, the sheer brutality of how he went about murdering particularly women and children is what shocked the world. Stalin didnt have extermination camps nor did he target specific ethnic groups/commit genocide. Even Mao killed about 80 million, 4 times as many people as Hitler killed, yet how often do we in the west view Mao as a monster? Stalin was evil but as he was an "allie" during WW2 that also deadens the attention as compared to Hitler, whom most people agree is the absolutely most evil human being to have ever existed.